Various arrangements have been proposed to vary the preload acting on a machine tool spindle for adjusting the bearing loads to accommodate different kinds of machining conditions. Many manufacturers now request that motor driven spindles be able to be field adjusted to at least three different preload levels.
In one such system it is necessary to remove a bearing set and include a different sized spacer within the set and then reassembling the bearing set and holding it in place by a bearing retainer arrangement threadably held in place on the end of the spindle. While able to establish different preloads on the bearings of the spindle, such assembly and disassembly of the bearing set components can cause undesirable delays in set-up time and in turn around time when the machines are reset for different operations requiring different preloading levels in each of a plurality of spindles connected with the machining operation. An example of replacement of components to vary bearing set preload in a spindle assembly is shown in U.S. Pat. No. 5,386,630.
Accordingly, various arrangements have been suggested for adjusting the preload on the bearing sets without requiring removal and replacement of various components in the bearing sets. One approach utilizes a mechanical screw arrangement for varying spindle bearing preload as shown in U.S. Pat. No. 4,657,412. Other approaches vary the spindle bearing preload by application of hydraulic pressure in response to various sensed operating conditions of the spindle are shown in U.S. Pat. Nos. 4,551,032; 5,388,917 and RE34,310.
While suitable for their intended purpose the aforesaid spindle bearing preload arrangements all require sensing systems or a continuous application of hydraulic pressure to maintain the desired adjusted preload condition.